Electrolyte for preparing porous coating with hierarchical structure on surface of titanium alloy by means of micro-arc oxidation

ABSTRACT

An electrolyte for preparing a porous coating having a hierarchical structure on the surface of a titanium alloy by means of micro-arc oxidation is disclosed. A TiO2 coating having high porosity and a hierarchical structure can be prepared on the surface of the titanium alloy by means of a micro-arc oxidation method based on the electrolyte of the present invention. The electrolyte consists of 0.07-0.15 mol/L tetraborate and 0.10-0.40 mol/L strong base. The tetraborate includes one of Li2B4O7, Na2B4O7 and K2B4O7. The strong base is selected from one of KOH and NaOH. The coating prepared from the electrolyte has a hierarchical structure consisting of interconnected micron grooves or micronpores and randomly distributed nanopores, a layered structure consisting of inner pores and outer slots, high porosity and super-hydrophilicity, and can be used for surface modification in the field of orthopedic materials, dental implants, etc.

TECHNICAL FIELD

The present invention belongs to the technical field of surface modification, and relates to a micro-arc oxidation electrolyte for preparing a titanium dioxide (TiO₂) coating on the surface of a titanium alloy.

BACKGROUND

Titanium and titanium alloys are widely used in human hard tissue implantation or substitution materials, but the surface of titanium and titanium alloys lacks bioactivity. Building surface microstructures through a surface modification technology is the main way to improve the surface bioactivity of titanium and titanium alloys.

Micro-arc oxidation (also known as anodic spark oxidation or plasma electrolytic oxidation) is a surface modification technology suitable for titanium and titanium alloys. Oxide coatings can be grown in situ on the surface of titanium and titanium alloys by means of micro-arc oxidation.

At present, most porous oxide coatings prepared on the surface of titanium and titanium alloys by means of micro-arc oxidation are of a crater-like or discrete porous structure. The pores of such surface structure are independent of each other, and lack of connectivity, so improvement in porosity and improvement in hydrophilicity are limited. Meanwhile, these coatings usually have monotonous structure scale and have no hierarchical structure feature. The interconnected pores or grooves and composite hierarchical structures of different scales as well as high hydrophilicity are all beneficial to improving the surface bioactivity of materials. In order to improve the connectivity and hydrophilicity of the microstructure and build a hierarchical structure at the same time, in the patent CN201210096780.5, a super-hydrophilicity cortex-like grooved TiO₂ coating is prepared on the surface of titanium by using tetraborate as an electrolyte. The coating has a hierarchical structure and super-hydrophilicity, so the bioactivity of the surface of titanium can be significantly improved.

Ti-6Al-4V (TC4 grade) and Ti-6Al-7Nb (TC20 grade) are commonly used titanium alloy materials. It is not enough to prepare a uniform cortex-like TiO₂ coating on the surface thereof by only using tetraborate as an electrolyte. In the present invention, on this basis, by adjusting the electrolyte component, a uniform cortex-like TiO₂ coating is prepared on the surface of Ti-6Al-4V and Ti-6Al-7Nb.

SUMMARY

In view of the limitation of the current titanium alloy micro-arc oxidation coating in bioactivity, the present invention proposes a TiO₂ micro-arc oxidation coating electrolyte. Based on the electrolyte, by adding strong base to a tetraborate electrolyte, a porous titanium dioxide coating having a hierarchical structure and high porosity can be prepared on the surface of a titanium alloy (for example, Ti-6Al-4V or Ti-6Al-7Nb) by means of micro-arc oxidation, so that the surface thereof is made to have high hydrophilicity, and the bioactivity of the surface thereof is improved.

To achieve the above purpose, the present invention adopts the following technical solution:

An electrolyte for preparing a porous coating on the surface of a titanium alloy by means of micro-arc oxidation. A TiO₂ coating can be prepared on the surface of the titanium alloy by means of a micro-arc oxidation method based on the electrolyte of the present invention, which includes two parts, i.e. solvent and solutes, wherein the solvent is deionized water; and the solutes are tetraborate and strong base, wherein the molar concentration of the tetraborate is 0.07-0.15 mol/L, and the molar concentration of the strong base is 0.10-0.40 mol/L.

The tetraborate is selected from one of Li₂B₄O₇, Na₂B₄O₇ and K₂B₄O₇, and the strong base is selected from one of KOH and NaOH.

A TiO₂ coating can be prepared on the surface of a titanium alloy by means of a micro-arc oxidation method based on the micro-arc oxidation electrolyte. The coating has uniformly distributed and interconnected micropores or grooves and has randomly distributed nanopores, and is a typical hierarchical structure surface. Moreover, the surface structure of the coating has certain inner pores to form a layered structure. The surface structure makes the coating have high porosity and super-hydrophilicity.

The micro-arc oxidation electrolyte of the present invention can be prepared by using a conventional method in the art, i.e. tetraborate and strong base are added to deionized water according to a ratio, uniformly stirred and fully dissolved.

The present invention has the following beneficial effects:

(1) Based on the micro-arc oxidation electrolyte of the present invention, a TiO2 coating can be prepared on the surface of a titanium alloy.

(2) The coating prepared on the surface of the titanium alloy based on the micro-arc oxidation electrolyte of the present invention has high porosity and super-hydrophilicity, has the hierarchical structure feature-uniformly distributed micropores or grooves and randomly distributed nanopores, has interconnected surface pores and grooves, and has certain inner pores and a layered structure.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a surface morphology of the coating prepared in embodiment 1 under a scanning electron microscope (3000×);

FIG. 2 shows a surface morphology of the coating prepared in embodiment 1 under a scanning electron microscope (5000×);

FIG. 3 shows an image of a water contact angle of the surface of the coating prepared in embodiment 1;

FIG. 4 shows a surface morphology of the coating prepared in embodiment 2 under a scanning electron microscope (5000×); and

FIG. 5 shows a surface morphology of the coating prepared in embodiment 3 under a scanning electron microscope (5000×).

DETAILED DESCRIPTION

The present invention will be further described below in combination with specific embodiments. The specific embodiments of the present invention and the description thereof are only used for explaining the present invention, not used for limiting scope of the present invention.

The polished Ti-6Al-4V or Ti-6Al-7Nb titanium alloy is used for micro-arc oxidation. The parameters of the micro-arc oxidation are as follows:

Power supply: micro-arc oxidation power supply;

Anode: polished titanium alloy plate (15 mm in length, 15 mm in width, and 2 mm in thickness);

Cathode: stainless steel electrolyzer;

Micro-arc oxidation control mode: constant voltage or constant current;

Micro-arc oxidation pulse mode: unipolar square pulse;

Micro-arc oxidation frequency: 600 Hz;

Micro-arc oxidation duty circle: 9%.

Embodiment 1

The polished Ti-6Al-4V is used as an anode. All components are weighed according to molar concentrations, and are mixed and stirred to be configured into an electrolyte. Solvent is deionized water; solutes are 0.10 mol/L of Na₂B₄O₇, and 0.25 mol/L of KOH; the power supply control mode is constant voltage, the set value is 300 V, and the treatment duration is 10 min. The image under the scanning electron microscopy shows that the coating has a hierarchical structure consisting of micron grooves and nanopores and a layered structure consisting of inner pores and outer slots; X-ray diffraction analysis shows that the prepared micro-arc oxidation coating is mainly made of rutile phase TiO₂ and anatase phase TiO₂; the energy spectrum shows that the atomic percentage of the aluminum element of the prepared micro-arc oxidation coating is 0.06%, and the atomic percentage of the vanadium element is 0.3%; and the contact angle test shows that the water contact angle of the prepared micro-arc oxidation coating is 10.2°, indicating that the coating has super-hydrophilicity.

Embodiment 2

The polished Ti-6Al-4V is used as an anode. All components are weighed according to molar concentrations, and are mixed and stirred to be configured into an electrolyte. Solvent is deionized water; solutes are 0.07 mol/L of Li₂B₄O₇, and 0.10 mol/L of NaOH; and the power supply control mode is constant current, the set value is 4 A, and the treatment duration is 30 min. The image under the scanning electron microscopy shows that the coating has a hierarchical structure consisting of micron grooves and nanopores and a layered structure consisting of inner pores and outer slots; X-ray diffraction analysis shows that the prepared micro-arc oxidation coating is mainly made of rutile phase TiO₂ and anatase phase TiO₂; the energy spectrum shows that the atomic percentage of the aluminum element of the prepared micro-arc oxidation coating is 0.64%, and the atomic percentage of the vanadium element is 0.52%; and the contact angle test shows that the water contact angle of the prepared micro-arc oxidation coating is 10.5°, indicating that the coating has super-hydrophilicity.

Embodiment 3

The polished Ti-6Al-7Nb is used as an anode. All components are weighed according to molar concentrations, and are mixed and stirred to be configured into an electrolyte. Solvent is deionized water; solutes are 0.15 mol/L of K₂B₄O₇, and 0.40 mol/L of KOH; the power supply control mode is constant current, the set value is 5 A, and the treatment duration is 10 min. The image under the scanning electron microscopy shows that the coating has a hierarchical structure consisting of micron grooves and nanopores and a layered structure consisting of inner pores and outer slots; X-ray diffraction analysis shows that the prepared micro-arc oxidation coating is mainly made of rutile phase TiO₂ and anatase phase TiO₂; the energy spectrum shows that the atomic percentage of the aluminum element of the prepared micro-arc oxidation coating is 0.05%, and the atomic percentage of the niobium element is 1%; and the contact angle test shows that the water contact angle of the prepared micro-arc oxidation coating is 9.3°, indicating that the coating has super-hydrophilicity.

The above embodiments illustrate that based on the present invention, a TiO2 micro-arc oxidation coating can be prepared on the surface of a Ti-6Al-4V or Ti-6Al-7Nb titanium alloy by adding a strong base to the tetraborate solution. The TiO2 coating prepared on the surface of the titanium alloy by means of the present invention has a hierarchical structure consisting of interconnected micro grooves or micropores and randomly distributed nanopores, and also has a layered structure consisting of inner pores and outer slots. The surface of the coating has high porosity, the surface structure is uniform in distribution, has certain connectivity between micropores, and has the feature of super-hydrophilicity.

Compared with previous studies, the TiO₂ coating prepared by means of the present invention has a hierarchical structure and a layered structure and the surface structure of the coating is uniform and flat, which can effectively improve the porosity, pore connectivity and hydrophilicity of the micro-arc oxidation coating, thereby improving the surface bioactivity of the titanium alloy.

The above embodiments only express the implementation of the present invention, and shall not be interpreted as a limitation to the scope of the patent for the present invention. It should be noted that, for those skilled in the art, several variations and improvements can also be made without departing from the concept of the present invention, all of which belong to the protection scope of the present invention. 

1. An electrolyte for preparing a porous coating on the surface of a titanium alloy by means of micro-arc oxidation, wherein a TiO₂ coating having high porosity and super-hydrophilicity and having a hierarchical structure can be prepared on the surface of the titanium alloy by means of a micro-arc oxidation method based on the electrolyte, wherein the coating has uniformly distributed and interconnected micropores or grooves; the electrolyte includes two parts, i.e. solvent and solutes, wherein the solvent is deionized water, and the solutes are tetraborate and strong base, wherein the molar concentration of the tetraborate is 0.07-0.15 mol/L, and the molar concentration of the strong base is 0.10-0.40 mol/L.
 2. The electrolyte for preparing a porous coating on the surface of a titanium alloy by means of micro-arc oxidation according to claim 1, wherein the tetraborate is selected from one of Li₂B₄O₇, Na₂B₄O₇ and K₂B₄O₇.
 3. The electrolyte for preparing a porous coating on the surface of a titanium alloy by means of micro-arc oxidation according to claim 2, wherein the strong base is selected from one of KOH and NaOH. 